Transistor detecting circuit

ABSTRACT

A transistor detecting circuit provided with a detecting sensitivity compensation transistor circuit having a compensation transistor and a resistance circuit connected between the base and collector of the compensation transistor, said compensation transistor circuit being provided in the input circuit leading to a detecting transistor.

United States Patent [191- Ichinohe et al.

[451 June 26, 1973 TRANSISTOR DETECTING CIRCUIT [75] Inventors: Eisuke Ichinohe, Kitakawachi-gun, Osaka; Noriyoshi Kubo, Asahi-ku, Yokohama; Kazuhiro Nakamura, Kohoku-ku, Yokohama; Yoshinori Endo, Midori-ku, Yokohama; Takeshi Katano, Kawasaki-shi, all of Japan [73] Assignee: Matsushita Electric Industrial Co.,

Lt.d, Osaka, Japan [22] Filed: Sept. 16, 1971 [21] App]. No.: 181,033

[30] Foreign Application Priority Data Sept. '18, 1970' Japan; 45/82355 [52] U.S.Cl. 307/310, 307/296 [51] lnt.Cl. H031: 17/00 [58] FieldofSearch 307/310, 296, 297;

[5 6] References Cited UNITED STATES PATENTS 2,892,165 6/1959 Lindsay 330/23 X 2,929,997 3/1960. Clawen 330/23 X 3,452,281 6/1969 Weischedel 330/23 X 3,585,511 Schatter 307/310 X Primary Examiner-Herman Kari Saalbach Assistant Examiner-B. P. Davis Attorney-Stevens, Davis, Miller & Mosher [57] ABSTRACT A transistor detecting circuit provided with a detecting sensitivity compensation transistor circuit having a compensation transistor and a resistance circuit connected between the base and collector of the compensation transistor, said compensation transistor circuit being provided in the input circuit leading to a detecting transistor.

2 Claims, 3 Drawing Figures 1 TRANSISTOR DETECTING CIRCUIT BACKGROUND OF THE INVENTION 1. Field of the Invention This invention relates to transistor detecting'circuits, and its aim is to obtain steady detection sensitivity irrespective of temperature changes.

2. Description of the Prior Art FIG. 1 shows a typical prior-art transistor detecting circuit, to which the invention pertains. It'comprises a detecting transistor TR a rectifying capacitor C,, a d-c block capacitor C a d-c clamp resistor R for clamping the dc component of the input signal and a collector load resistor R In this circuit, if Z,,, R, where Z is the impedance with respect to the transistor TR, and R, is the input impedance of a signal source E, the condition for the input signal e from the signal source E to be detected as it is impressed across the base and emitter of-the detecting transistor TR is expressed as where V is the potential difference between the base and emitter of the transistor TR The voltage V is subject to changes with changing temperature. This means that the detection sensitivity of the transistor detecting circuit of the above construction also changes with a change in temperature.

SUMMARY OF THE INVENTION An object of the invention is to provide a transistor detecting circuit provided with a detection sensitivity compensation transistor circuit consisting of a transistor and a resistance circuit connected between the base and collector of the transistor, said transistor circuit being provided in the input circuit leading to the detecting transistor, thereby ensuring steady detection sensitivity irrespective of temperature changes.

BRIEF DESCRIPTION OF THE DRAWING FIG. 1 is a circuit diagram showing a conventional transistor detecting circuit.

FIG. 2 is a circuit diagram showing an embodiment of the transistor detecting circuit according to the invention.

FIG. 3 is a graph showing the detection sensitivity characteristic of the circuit FIG. 2. I

DESCRIPTION OF THE PREFERRED EMBODIMENT FIG. 2 shows an embodiment of the transistor detecting circuit according to the invention. This circuit includes a transistor circuit which consists of a transistor TR, and resistors R,, R and R and which replaces the resistor R in the prior-art circuit of FIG. I.

In this circuit construction, if R,I, V, where R is the resistance of the resistor R 1, is the current therethrough and V is the potential difference between the base and emitter of the transistor TR R,! can be regarded as a constant current source.

For the input signal e to be detected by the transistor TR,, the following condition is required.

.This relation is illustrated in FIG. 3.

The inequality 2 is based on the assumption that the resistor R offers infinitely high resistance and carries no current.

In the absence of the input signal e to the transistor TR the transistor TR, is cut off providing no base current, so that no voltage drop is produced across the resistor R In this state, the potential at point A is equal to the collector potential of the transistor TR so that the inequality 2 is satisfied.

If V V the inequality 2 reduces to As is apparent from the inequality 3, the detection sensitivity does not depend upon the base-emitter voltage of the transistors TR and TR Since R in the inequality 3 is constant, the detection sensitivity solely depends upon I Since I, (V V Q/R and I, [:(l 1/3) where [3 is the current gain of the transistor TR Substitution of the equation 4 to the inequality 2 gives If B l, the denominator of the second factor '(V V /l 1/3) on the right side of the inequality 5 is substantially at unity, so that the detection sensitivity is not affected by [3.

Also, if R and R fluctuate in the same direction, their effects cancel and have no effect on the detection sensitivity.

If V V fluctuation of the base-emitter voltage V of the transistor TR has no effect.

Further, it will be apparent from the inequality 5 that the detection sensitivity can be varied by varying the resistances R and R,.

As has been described, according to the invention it is possible to obtain a transistor detecting circuit having a steady sensitivity characteristic immune to the influence of temperature.

We claim;

l. A temperature-compensated detecting circuit comprising a detecting transistor having its emitter and collecto electrodes coupled between an output terminal and ground, the base electrode of said detecting transistor being coupled to an input terminal for receiving an input signal,

a series circuit comprising a compensation transistor and a first resistor connected between the base of said detecting transistor and ground, said first resis e an: 2 4 an,

where e is the input signal to be detected, V is the potential difference between the base and the emitter of said compensation transistor, V is the potential differ ence between the base and the emitter of said detecting 

1. A temperature-compensated detecting circuit comprising a detecting transistor having its emitter and collector electrodes coupled between an output terminal and ground, the base electrode of said detecting transistor being coupled to an input terminal for receiving an input signal, a series circuit comprising a compensation transistor and a first resistor connected between the base of said detecting transistor and ground, said first resistor having a relatively large resistance value, means for providing a constant current to the base of said compensation transistor, and a second resistor connected between the base and collector electrodes of said compensation transistor for coupling therewith.
 2. A detecting circuit according to claim 1 wherein detection is performed under the condition e + VBE2 - I2R4 > or = VBE, where e is the input signal to be detected, VBE2 is the potential difference between the base and the emitter of said compensation transistor, VBE is the potential difference between the base and the emitter of said detecting transistor, R4 is the resistance of said second resistor connected between the base and the collector of said compensation transistor, and I2 is the current through resistor R4. 